The preset invention relates to a device and method for carrying out an air-fuel ratio feedback control of an internal combustion engine, and more specifically, to a technology for carrying out a feedback control using a sliding mode control.
It is common to carry out a feedback control for an internal combustion engine for vehicle, so as to approximate an air-fuel ratio to a target value, in order to improve the fuel consumption or the exhaust emission.
Therefore, while detecting the air-fuel ratio sequentially by an air-fuel ratio sensor equipped in an exhaust passage and the like, the fuel supply quantity is feedback controlled using PID control (proportional-integral-derivative), in order to converge the detected air-fuel ratio to the target air-fuel ratio.
On the other hand, a sliding mode control is known as a control method having high robust performance with suppressed influence from disturbance, which is often used in controlling robots and the like. A proposal is made to utilize the sliding mode control to the feedback control of the air-fuel ratio (Japanese Unexamined Patent Publication No. 8-232713).
However, the sliding mode control mentioned above used for the feedback control of the air-fuel ratio is not capable of eliminating the influence caused by the dispersion in the part performance for each engine, and therefore, was not highly accurate. This is because in designing the sliding mode control, the air-fuel ratio control system of the engine is modeled considering the response delay and the like of each part.
Moreover, the air-fuel ratio feedback control using the sliding mode control has a general problem. Since the air-fuel ratio is detected through a specific component in the exhaust, dead time, such as transfer delay of the exhaust and the like, exists between the air-fuel ratio detected from the exhaust and the actually controlled air-fuel ratio (fuel supply quantity). During such dead time, excessive compensation is performed. If such dead time is fixed, the computing cycle of the feedback control amount can be set corresponding to such a fixed dead time, to restrain excessive correction. However, the dead time greatly varies according to the operating conditions of the engine, and therefore, it was very difficult to set the computing cycle to an appropriate value. For example, if the computing cycle is set sufficiently large, excessive correction can be restrained, but the response characteristic is reduced. Actually, the computing cycle is set to be small to a certain extent to restrain excessive correction by keeping the feedback gain small. However, this deteriorates the response characteristic, and prevents the sliding mode control from performing its function sufficiently.
Heretofore, the sliding mode control utilized in the air-fuel ratio feedback control involves modeling and designing in detail the delay of various units in the air-fuel ratio control system including the above-mentioned dead time. Therefore, it involved extremely complicated and bothersome processes, could not be used generally for different types of vehicles or engines, and needed large capacity of ROM and RAM for carrying out the complicated control.
The present invention aims at solving the above mentioned problems. The object of the invention is to provide an air-fuel ratio feedback control using a highly accurate sliding mode control without no dispersion for each engine, that can be carried out easily, that is easy to design, and that can be generally applied to various types of vehicles and engines.
Another object of the invention is to ensure a good response characteristic while restraining excessive correction during dead time in an air-fuel ratio feedback control according to a sliding mode control.
Yet another object of the invention is to eliminate the influence of the dead time element existing in the control object without depending on the later correction of the feedback control amount, thereby ensuring the stability the response characteristic of the control system.
In order to achieve the first common object, the present invention includes the following basic constitution.
An air-fuel ratio is detected linearly by an air-fuel ratio sensor equipped for example in the exhaust passage.
A feedback control amount is computed according to a sliding mode control in which a deviation between a target air-fuel ratio set according to operating conditions of the engine and the detected air-fuel ratio is set as a switching function.
A feedback control is carried out using the computed feedback control amount, so as to approximate the detected air-fuel ratio to the target air-fuel ratio.
According to this constitution, the feedback control of the air-fuel ratio is carried out using the feedback control amount according to the sliding mode control in which the deviation (error) between the target air-fuel ratio and the detected air-fuel ratio (actual air-fuel ratio) is set as a switching function S. Thereby, the air-fuel ratio converges to the target air-fuel ratio while sliding along a switching plane defined as S=0 (in other words, error=0).
Here, the switching function S is set through a method called the direct switching function method of the sliding mode control. This method defines the switching plane (S=0) as a function representing the state to be achieved (in this case, to approximate the air-fuel ratio to the target air-fuel ratio). This method is characterized in that though there is no assurance that the state will slide on the switching plane, once the sliding is confirmed, it enables to provide the best sliding mode control. This is because the switching plane is decided based only on whether the target value is greater or smaller than the actual value, irrespective of a change in response characteristic of a fuel supply device or the air-fuel ratio sensor and the like.
When sliding mode control of the air-fuel ratio is carried out using the switching function set as explained above, it is confirmed that the air-fuel ratio slides along the switching plane.
Therefore, the feedback control according to the sliding mode control can be carried out easily and with high accuracy.
Moreover, unlike the conventional art, the present invention does not involve the complicated process of modeling the engine in order to set the switching function. Therefore, the present invention can be applied generally to different types of vehicle or engine.
Even further, the feedback control amount may include a linear term and a nonlinear term.
According to such a constitution, the feedback control amount computed by the sliding mode control comprises a linear term and a nonlinear term, the linear term adjusting the speed for approximating the state of the control system to the switching plane, and the nonlinear term generating the sliding mode along the switching plane.
Moreover, the linear term may be computed as a value proportional to the ratio between the deviation of the target air-fuel ratio and the detected air-fuel ratio to the detected air-fuel ratio.
According to this constitution, as the air-fuel ratio separates from the switching plane, in proportion to this separation, the linear term is set to a greater value, enabling the air-fuel ratio to approximate the switching plane promptly while suppressing overshooting.
Further, the nonlinear term may be computed by integrating a feedback gain, the positive or negative of which is switched according to whether the switching function is positive or negative.
According to this constitution, the positive or negative of the switching function will reverse whenever the state of the air-fuel ratio crosses the switching plane, and the positive or negative of the feedback gain will also reverse accordingly. By the nonlinear term computed by integrating the feedback gain, the air-fuel ratio promptly converges to the target air-fuel ratio while sliding along the switching plane.
Moreover, the absolute value of the feedback gain may be set to vary according to the operating conditions of the engine.
The delay time until an air-fuel ratio of an intake air being the control object is detected as an air-fuel ratio of the exhaust by an air-fuel ratio detecting means, varies according to the operating conditions of the engine. Therefore, by setting the feedback gain variably according to the operating conditions of the engine, the value of the nonlinear term integrated during the delay time can be fixed. Thus, while the response characteristic is ensured (the response characteristic will be deteriorated if the feedback gain is reduced uniformly), the deviation of the actual air-fuel ratio from the target air-fuel ratio can be appropriately reduced, to realize a stable air-fuel ratio feedback control.
Moreover, the absolute value of the feedback gain may be set to a greater value as an intake air quantity increases.
In this way, since the flow of the exhaust is small when the intake air quantity is small, the time needed for the exhaust from a cylinder to reach the air-fuel ratio detecting means is short, and the contact pressure of the exhaust to the air-fuel ratio detecting means is small. Therefore, the delay time for the air-fuel ratio of the exhaust to be detected is increased. Accordingly, the absolute value of the feedback gain can be set to a greater value as the intake air quantity increases, to fix the value of the nonlinear term integrated during delay time.
Moreover, the absolute value of the feedback gain may be set to increase as the engine rotation speed increases.
When the engine rotation speed is low, the exhaust from the cylinder reaches the air-fuel ratio detecting means in a short time, and so the delay time for the air-fuel ratio of the exhaust to be detected is increased. Therefore, the absolute value of the feedback gain is set to a greater value as the engine rotation speed increases, thereby enabling to fix the value of the nonlinear term integrated during delay time.
Moreover, the air-fuel ratio may be detected by a wide-range air-fuel ratio sensor that detects the air-fuel ratio linearly based on a specific component in the exhaust.
According to this constitution, the wide-range air-fuel ratio sensor can detect the air-fuel ratio highly accurately, which leads to carrying out a highly accurate air-fuel ratio feedback control according to the sliding mode control.
Alternatively, the air-fuel ratio may be detected by a narrow-range air-fuel ratio sensor that detects the rich/lean of the air-fuel ratio in an on/off manner based on a specific component in the exhaust, and the value detected by the narrow-range air-fuel ratio sensor is linearized and then utilized as the detection value of the air-fuel ratio to compute the feedback control amount.
According to such a constitution, even when a narrow-range air-fuel ratio sensor detecting the rich/lean of the air-fuel ratio in an on/off manner is utilized by linearizing the value detected by the narrow-range air-fuel ratio sensor, the linearized air-fuel ratio detected value can be used to perform the air-fuel ratio feedback control using the sliding mode control according to the present invention.
Next, the present invention for achieving the second object, in which by the computing cycle of the feedback control amount, high response characteristic is ensured while restraining excessive correction during dead time, includes the following constitution in addition to the basic invention.
That is, the computing cycle is controlled so that the feedback control amount according to the sliding mode control is computed in synchronism with the stroke cycle of the engine.
As mentioned, the dead time for the exhaust discharged from the combustion chamber of each cylinder to reach the air-fuel ratio detecting means changes greatly depending on operating conditions. However, if the dead time is converted to the number of strokes of the engine, most is fixed without much deviation. In other words, the exhaust discharged from a combustion chamber during the exhaust stroke of one cylinder is presumed to reach the air-fuel ratio detecting means during a plural number of times of the exhaust stroke of other cylinders. Therefore, by controlling the computing cycle so that the feedback control amount of the sliding mode control is computed in synchronism with the stroke cycle of the engine, the number of times of computing during dead time is withheld to below a fixed value or below, and excessive correction can be restrained. Moreover, as a result, the feedback gain could be increased sufficiently, leading to high response characteristic.
The computing cycle for the feedback control amount may be controlled to correspond to a multiplicative value of the stroke cycle of the engine.
Most simply, even if the feedback control amount can be computed per one stroke cycle of the engine, the excessive correction can be restrained to a certain number of times of the stroke cycles or below. However, excessive correction could be further restrained if the computing cycle corresponds to a multiplicative of the dead time converted into the number of stroke cycles.
Further, the computing cycle of the feedback control amount may be variably controlled according to the operating conditions of the engine.
Though the value obtained by converting the dead time with the number of stroke cycles is mostly fixed, it may be considered to vary according to operating conditions. Therefore, the computing cycle of the feedback control amount is controlled variably according to the operating conditions of the engine, resulting in more accurate control.
For example, when considering how many times of exhaust discharged from each cylinder are required in order to fill the capacity of the exhaust passage from the combustion chamber to the air-fuel ratio detecting means (sensor), it depends on the amount of exhaust for each cylinder. Therefore, by variably controlling the computing cycle, that is a multiplicative of the stroke cycle, through a value representing the amount of exhaust, such as a basic fuel injection quantity Tp and the like, a more accurate control can be carried out.
Moreover, the computing cycle of the feedback control amount may be variably controlled according to the deviation between the target air-fuel ratio and the actual air-fuel ratio detected by the air-fuel ratio detecting means.
According to this constitution, when the deviation is large, the computing cycle, which is the multiplicative of the stroke cycle, is reduced by making much of the response characteristic (reduce the multiplicative value), and at the time when the deviation is reduced, the computing cycle is increased to a cycle (multiplicative of the stroke cycle) corresponding to the dead time, so as to carry out the control with the reduced deviation from the target air-fuel ratio.
Even further, the computing cycle may be controlled so that the feedback control amount is computed in synchronism with a signal detecting a predetermined stroke timing of each cylinder.
According to this constitution, the feedback control amount is computed in synchronism with the signal detecting the predetermined stroke timing of each cylinder, so that the computing cycle can be easily synchronized with the stroke cycle of the engine.
Moreover, a reference signal output from a crank angle sensor for detecting the rotation speed or determining the cylinders at a predetermined stroke timing of each cylinder may be used as the signal detecting the predetermined stroke timing of each cylinder. Therefore, the computing timing of the feedback control amount can easily be controlled.
Alternatively, a detection signal detecting the inner-cylinder pressure used for detecting knocking and the like may be used as the predetermined stroke timing of each cylinder. The predetermined stroke timing can be detected for example by using the fact that the inner-cylinder pressure increases at the top dead center of compression.
Next, the preset invention for achieving the third object, in which the influence of the dead time element existing in the control object can be eliminated without depending on the later correction of the feedback control amount, thereby ensuring the stability and the response characteristic of the control system, is realized by the following constitution.
The air-fuel ratio is detected linearly by an air-fuel ratio sensor and the like provided in the exhaust passage.
The phase delay caused by a dead time element of the control object included in the detected air-fuel ratio is compensated for using a model of the control object represented by a transfer function that is switched according to the operating conditions of the engine.
The feedback control amount is computed using the detected air-fuel ratio whose phase delay has been compensated for, based on a sliding mode control in which the deviation between the target air-fuel ratio and the actual air-fuel ratio detected by the air-fuel ration detecting means is set as a switching function.
According to such constitution, the phase delay of the air-fuel ratio detected by the air-fuel ratio sensor is compensated for before the feedback control amount is computed. Thus, the feedback control amount is computed based on the detected air-fuel ratio after being compensated for its delay. Moreover, since the transfer function representing the control object model can be switched depending on the operating conditions of the engine, a model corresponding to every operating condition of the engine can be used to correspond to the various properties of the control object, such as the filling of the gas to the cylinder, the adhesion of the fuel from the fuel supply device to the wall surface of the intake pipe, and the transfer of the exhaust.
In this way, the feedback control amount can be computed to become most suitably when eliminating the influence of the dead time element existing in the control object. Thus, a large feedback control amount can be output throughout the whole region of the engine operating condition while maintaining the stability of the control system, and therefore, a good response characteristic of the control system can be ensured. Even further, since a model corresponding to the property of the control object is used, the phase delay of the detected air-fuel ratio can be accurately compensated for without depending on the change of property of the control object, thereby improving the robustness of the control system.
Moreover, the model of the control object may be composed of a first element that does not include the dead time element, and a second element that represents the dead time element.
According to such constitution, the influence of the dead time element existing in the control object is eliminated using the model representing the second element.
The feedback control amount is computed based on the control object eliminated of such dead time element, or ideally, an output that passed through only the transfer function representing the first element.
In this way, the phase delay of the detected air-fuel ratio can easily be compensated for using the second element, and the feedback control amount can be computed easily based on the information that does not include any phase delay.
Further, a Smith dead time compensation control (proposed by Otto Smith) may be used to compensate for the phase delay included in the detected air-fuel ratio.
According to this constitution, the influence of the dead time element existing in the control object is apparently excluded from the feedback group using the model representing the second element. The feedback control amount is computed based on the output obtained from prior to the excluded dead time element, or ideally, the output that passed through only the transfer function representing the first element.
Thus, the phase delay of the detected air-fuel ratio is compensated easily, and the air-fuel ratio feedback control is carried out by a more simple control system.
Moreover, in the transfer function representing the model of the control object, the order of the transfer function representing the first element may be varied and switched. In other words, the order of the transfer function representing the first element, or the formula of the transfer function itself, may be switched, so as to correspond to the change (increase and decrease) in the variety of response delay elements causing the response delay of the control object, such as the delay in the filling of gas in the cylinder or the adhesion of the fuel from the fuel supply device to the wall surface of the intake pipe.
Thereby, the phase delay of the detected air-fuel ratio can be correctly compensated without depending on the change in the variety of response delay elements existing in the control object, and at the same time, the present constitution can contribute to a more accurate computing of the feedback control amount.
Moreover, in the transfer function representing the model of the control object, the time constant of the transfer function representing the first element may be varied and switched. In other words, the time constant of the transfer function representing the first element may be able to change in order to correspond to the change in property of the response delay element existing in the control object.
Thereby, the phase delay of the detected air-fuel ratio could be compensated accurately without depending on the change of property of the response delay element existing in the control object, and therefore, the present constitution can contribute to a more accurate computing of the feedback control amount.
Even further, in the transfer function representing the model of the control object, the dead time of the transfer function representing the second element may be varied and switched. In other words, the dead time of the transfer function representing the second element may be able to change in order to correspond to the change in property of the dead time element existing in the control object, especially the change in dead time caused by the transfer delay of the exhaust.
According to this constitution, the phase delay of the detected air-fuel ratio can be compensated accurately without depending on the dead time element existing in the control object, especially the change in dead time caused by the transfer delay of the exhaust.
Even further, the transfer function may be switched according to the acceleration or deceleration of the engine.
According to this constitution, the control object model can be switched according to the change in property of the control object caused by the acceleration or deceleration of the engine.
Thereby, the transfer function representing the control object model, and further the control object model itself, can be easily switched according to the change in the property of the control object based on the acceleration or deceleration of the engine, and the best control object model corresponding to the property of the control object can be used.
Even further, the transfer function may be switched according to the intake air quantity.
According to this constitution, the control object model can be switched according to the change in property of the control object caused by the change in the intake air quantity.
Thereby, the transfer function representing the control object model, and further the control object model itself, can be easily switched according to the change in the property of the control object based on the intake air quantity, and the best control object model corresponding to the property of the control object can be used.
Even further, the transfer function may be switched according to the wall temperature of the intake pipe.
According to this constitution, the control object model can be switched according to the change in property of the control object caused by the change in the intake pipe wall temperature.
Thereby, the transfer function representing the control object model, and further the control object model itself, can be easily switched according to the change in property of the control object based on the intake pipe wall temperature, and the best control object model corresponding to the property of the control object can be used.
The other objects and features of this invention will become understood from the following description with reference to accompanying drawings.